The present invention relates to a control circuit for speeding up the turn-off time of a PNP transistor. In particular, the embodiments of the invention include a circuit that reduces both the delay to the start of the transition time and the transition time from an on state to an off state of a PNP transistor. This application is related to the inventor's copending application entitled "Lateral PNP Fast Turn-On Circuit" having the same filing date as the present application.
PNP transistors are often employed as switching elements in integrated circuits. Typically, during normal switching, the PNP transistor is cycled "on" and "off" to control the flow of current between the emitter and collector by intermittently supplying a forward biasing or drive current to the transistor. The transistor is turned "on" by base drive current and turned "off" by removing the base drive current. During "on" periods, an amount of charge is stored in the base of the transistor. When the drive current is removed, the emitter base junction of the PNP transistor is caused to remain forward biased for a period of time by the stored charge, and the transistor continues to conduct current between the emitter and collector during the time required for the stored charge to decay. The stored charge increases the time for the transistor to transition from a conducting state to a nonconducting state, resulting in delayed switching. When the transistor is driven into saturation by the base drive, the charge stored in the base must be discharged before any reduction in the current conducted between the emitter and collector of the transistor can take place following the removal of the drive current. The delay in transition time and the transition time itself may each be up to several microseconds long. These relatively long time periods reduce the maximum switching speed of the PNP transistor.
The discharge period of the stored charge can be reduced by applying a reverse drive current to the base of the switching PNP transistor. The greater the level of reverse drive current, the faster the base stored charge will be discharged and the faster the transistor will turn "off." To drive the PNP transistor quickly into a nonconducting state, the reverse drive current must be able to drive the base voltage of the PNP transistor close to the level of the voltage at its emitter.
Circuits have been developed in the past for decreasing the switching time of a transistor. For example, U.S. Pat. No. 4,487,520 (O'Neill et al.) and U.S. Pat. No. 5,128,553 (Nelson) both describe circuits which generate a reverse drive current to speed up the switching speed of a PNP transistor from the "on" state to the "off" state. These two patents disclose the use of an NPN transistor to provide a reverse drive current to the base of a PNP switching transistor. In certain circuit environments such as in automotive uses, the output at the collector of the PNP transistor may be subjected to high voltage transients. Voltage transients of up to .+-.40 volts may be found in an automotive environment for example. The circuits disclosed in the patents to Nelson and O'Neill et al. can only withstand a positive voltage differential from output to the positive voltage supply V+ of about 8-9 volts without damage to the circuitry. The relevant portions of the circuitry in these two patents subjecting them to vulnerability to large positive voltage differentials is the same. The emitter of an NPN drive transistor is connected to the base of the PNP switching transistor. The collector of the NPN transistor is tied to the positive voltage supply V+ and the base is connected to the cathode of a diode. As the voltage at the collector of the PNP transistor increases with respect to the positive voltage supply V+ due to a transient event, the path for the current to flow from the collector of the PNP transistor to the positive voltage supply is through the forward-biased collector-base junction of the PNP switching transistor and then through the reverse-biased emitter-base junction of the NPN drive transistor and finally through the forward-biased base-collector junction of the NPN transistor to the positive supply voltage V+. The two forward-biased junctions have a voltage drop across them of about 0.7 volts each and the breakdown voltage of the emitter-base junction of the NPN drive transistor in this type of bipolar technology is about 7 volts. This total voltage drop is thus about 8.4 volts. If the transient voltage on the output terminal at the collector of the PNP transistor rises above the positive voltage supply V+ by a level greater than this 8.4 volts, the emitter-base junction of the NPN transistor will fail catastrophically. The path for the current in this situation has no limiting resistance in the path so the weakest link, the emitter-base junction, will overheat from the high current and fail catastrophically.